Field of the Invention
The present invention relates to an image forming apparatus, an image forming system, and an image formation control method, and, in particular, relates to a technique to execute control efficiently in the case of forming images repeatedly on a long sheet such as a rolled sheet.
Description of Related Art
In image forming apparatuses, generally, images are formed on a sheet called a cut sheet which is cut in a predetermined fixed sheet size, such as a letter size. On the other hand, in image forming apparatuses, images can be also formed on a long sheet called a rolled sheet. Such a long sheet has a length of about several hundred meters or several kilometers, and, in many cases, the long sheet is used for a purpose of printing a label repeatedly.
In image forming apparatuses of a electrophotographying system, processing (image stabilization processing) for stabilizing image density is generally performed periodically.
For example, in an image forming apparatus which uses sheets of a fixed paper size, a patch image with a prescribed density (optical density) is drawn on a transfer member, such as an intermediate transfer belt with a timing within an interval between a sheet and a sheet. Then, the density of this patch image is read by a density sensor, and an image forming section executes correction based on the density so as to enable image formation with stable density.
Further, in such an image forming apparatus of an electrophotographying system, when toner to be used for image formation becomes old, there is a control technique to form a toner consumption patch image so as to discard the old toner positively. In this case, the discarding of the old toner enables stable image formation.
However, in the case of image formation by using a long sheet, there is no space between sheets and there exists a sheet always in an image forming apparatus. Accordingly, if a patch image is drawn, the patch image is made to be transferred at some position on a sheet. Therefore, in order to execute image stabilization processing, it is necessary to secure a region on a sheet where a patch image is drawn.
However, in image formation by using a long sheet, in many cases, a gap in a sub scanning direction (sheet conveying direction) between an image and another image is about 1 mm. Accordingly, it is difficult to secure a region where a patch image is drawn.
Then, in order to draw a patch image, a technique to stop image output in the sub scanning direction for a predetermined period may be considered. However, in this case, a cycle from an image to a next image is made to change.
Therefore, a post processing apparatus connected to a back stage of an image forming apparatus is required to perform control in consideration of a patch image cycle, which results in that the control required for the post processing apparatus becomes complicate. In this case, with a sudden control change of the post processing apparatus, there is a possibility that the quality of an output matter may deteriorate. Further, there is also a problem that waste sheets may increase correspondingly to the stop of image formation.
With regard to control in image forming apparatuses which use a long sheet or a continuous sheet, various kinds of proposals have been made by Japanese Unexamined Patent Publication Nos. 2013-132847 (Document 1) and 2006-84796 (Document 2).